1. Field of the Invention
The present invention relates to a bar-shaped light guide, a line-illuminating device incorporated with the light guide, and a contact-type image sensor incorporated with the line-illuminating device.
2. Description of the Prior Art
A contact-type image sensor has a smaller number of parts than an image sensor using a reducing optical system. A sensor and a lens array which are optical components can also be arranged closely. Accordingly, there is an advantage that the contact-type image sensor can be made comparatively thin. Thus, the contact-type image sensor is used as equipment for reading a document in a facsimile machine, a copying machine, a hand scanner and the like.
The contact-type image sensor is provided with a line-illuminating device for linearly illuminating a document-reading surface along the main scanning direction. The line-illuminating device using the light guide is known.
FIG. 22 is a cross-sectional view of a contact-type image sensor developed by the present inventor et al. and FIG. 23 is a perspective view of a light guide which is used in that contact-type image sensor.
The contact-type image sensor 101 is provided with a frame 102 in which a line-illuminating device 110 is installed. A lens array 105 is arranged within the frame 102. Mounted on the lower part of the frame 102 is a base plate 107 which is provided with a line image sensor (i.e. a photoelectric conversion element) 106. The line-illuminating device 110 consists of a light guide 103, a light guide casing 104, and a light-emitting source base plate provided with an LED or the like (not shown).
The contact-type image sensor 101 allows illuminating light emitted from an emission plane 103a of the light guide 103 to be incident on a reading surface of a document through a cover glass 108. The document is read by detecting the reflected light using the line image sensor 106 through the lens array 105.
The light guide 103 is made of glass or transparent resin and as shown in FIG. 23, it has a cross-sectional shape substantially xc2xc oval in the direction perpendicular to the longitudinal direction. A side surface of the light guide 103 along the longitudinal direction is provided with the emission plane 103a parallel to a minor axis direction of the oval, a plane 103b parallel to the major axis direction of the oval, and a reflecting curved plane 103c. Light scattering patterns 103P are formed on the plane 103b parallel to the major axis direction of the oval by printing white coating materials. The light guide 103 allows the illuminating light incident from end surfaces in the longitudinal direction to be reflected by the inner surface and guides the reflected light in the longitudinal direction. The light guide 103 also allows the light scattered by the light scattering patterns 103P to be reflected by the reflecting curved plane 103c and to emit the reflected light from the emission plane 103a. Now, by changing an area of the light scattering patterns 103P in response to a distance from the end surfaces of the light guide 103, it is intended that the intensity of light emitted from the emission plane 103a be uniform along the longitudinal direction of the light guide 103.
The light guide 103 is, as shown in FIG. 22, housed in the white light guide casing 104 in such a manner that the emission plane 103a is exposed. In this manner, by covering the light guide 103 using the light guide casing 104, the light emitted outside is caused to reflect by the light guide casing 104 and returned to the inside of the light guide 103. Thus, loss of the scattered light is reduced and as a result, intensity of the emission light is improved. The light scattering patterns 103P are formed at a position near a focal point on the plane 103b parallel to the major axis direction of the oval. With this, the light scattered by the light scattering patterns 103P is reflected by the reflecting curved plane 103c and is condensed on the document reading surface. As a result, it is possible to improve the intensity of light on the document-reading surface.
The light emitting source base plate (not shown) provided with a light emitting source consisting of an LED or the like is attached to one end or both ends of the light guide casing 104. In this manner, the line-illuminating device comprises the light guide 103, the light guide casing 104, and the light emitting source base plate.
The conventional light guide and line-illuminating device incorporated with the light guide described above have a first problem described below.
According to the conventional device, it is intended that quantity of illuminating light be maximum in a condition in which the document surface is not elevated. Accordingly, when the document surface is elevated by a fold or spread of the document, the quantity of illuminating light decreases, and an unnatural shade is caused on an image read by the line image sensor 106.
FIG. 24 is a graph showing light intensity characteristics of the contact-type image sensor described above. In FIG. 24, the horizontal axis shows displacement of a sub-scanning direction of which the origin is the focal distance (which is on an upper surface of the cover glass plate 108) of the lens array 105 and the vertical axis shows light intensity measured at each point.
Parameter of each curve means a distance upward from the surface of the glass plate (e.g. the distance 0.0 mm is the surface of the glass plate). Namely, data shown in FIG. 24 is not that measured using a line sensor of the device, but that measured by a separate sensor provided above the glass plate. Accordingly, xe2x80x9clight output characteristics when the document surface is elevated by x mmxe2x80x9d specifically means xe2x80x9cthe light output characteristics measured by an optical sensor which is put at a distance of x mm above the upper side surface of the glass plate, corresponding to a state where the document surface is elevated by x mmxe2x80x9d. However, this is described below as xe2x80x9cthe light output characteristics when the document surface is elevated by x mmxe2x80x9d for clarification.
In FIG. 24, a position in which light intensity is maximum in a condition where the document surface is not elevated (i.e. the position where the illuminating light emitted from the light guide 103 is converged on the document surface) is set as the origin 0 of the displacement in the sub-scanning direction. A side of the light guide 103 from this position is shown by a minus value, while the opposite side is shown by a plus value. A black round mark indicates the light output characteristics when the document surface contacts the cover glass 108 and a square mark indicates the light output characteristics when the document surface is elevated by 0.5 mm from the cover glass 108. A triangular mark indicates the light output characteristics when the document surface is elevated by 1 mm and an x mark indicates the light output characteristics when the document surface is elevated by 1.5 mm. A round mark indicates the light output characteristics when the document surface is elevated by 2.0 mm.
In the contact-type image sensor described above, an optical axis of the lens array 105 is arranged in a position where the displacement in the sub-scanning direction is the origin 0, wherein a light-receiving surface of the line image sensor 106 is arranged on the optical axis. Thus, as shown in FIG. 24, when the document is elevated by 1 mm, the quantity of illuminating light (i.e. quantity of reading light) decreases by 20% or more. Accordingly, unevenness in illumination is produced at a position where bending, crinkles, fold, or spread occur on the document surface and as a result, an embossed-like image has been produced even on a simple white document in an extreme case.
If the reflecting curved plane (i.e. the oval plane) of the light guide is sufficiently increased, the scattering patterns are relatively decreased. Accordingly, an image of the scattering patterns can be formed on the optical axis of a rod lens and an ideal light intensity distribution can be obtained. However, the contact-type image sensor must be larger.
The conventional line-illuminating device also has a second problem described below.
Namely, to improve efficiency of condensing onto an illuminating surface (i.e. a document surface) using the light guide with an oval cross-sectional shape, it is necessary to deepen the oval plane so as to reflect and condense the scattered light more from the focal position. However, in this case, since the cross-sectional area of the light guide increases, there is a trade-off (a problem) in that density of propagation light on the inside of the light guide decreases and intensity of the scattered light generated by the light scattering patterns decreases.
According to the conventional light guide, a single member is provided to fulfill a light guiding and scattering function for guiding the light incident from the end surfaces in the longitudinal direction and generating the scattered light by the light reflecting patterns, and a light condensing function for allowing the scattered light to be reflected by the reflecting curved plane (i.e. the oval plane) to emit it from the emission plane, thereby allowing the emission light to be condensed on the document surface. Accordingly, it is difficult to independently improve the light guide and scattering efficiency and the light condensing efficiency.
Further, to increase the quantity of the emission light, a method for broadening an area of the light scattering patterns has been adopted in the prior art. However, linear dimensions (i.e. dimensions in a main-scanning direction) of the light scattering patterns are limited because uniformity of the light intensity in the main-scanning direction must be secured. This can be considered to increase the width of the light scattering patterns, but the flatter the oval, the more the scattering of light to a point other than a light condensing point is increased. Accordingly, this does not contribute to the increase of light quantity.
Even though the width of the light scattering patterns P is increased, the scattered light from a position where it is displaced a lot from a focal point of the oval is not condensed on an intersection point between the document reading surface and the optical axis of the lens array, but the scattered light diffuses. As a result, the light intensity at the intersection point tends to decrease.
On the other hand, to precisely allow the light to be condensed on the light condensing point (i.e. the document reading surface directly above the optical axis of the lens array), it is desirable that the oval be not as flat (i.e. the oval be close to a circle) and the dimension be larger. The oval which is not so flat is more tolerant to the displacement of the focal position of the scattered light and contributes to the increase of light quantity at the light condensing point to a certain extent even though the width of the light scattering patterns is increased. However, if the oval is closer to the circle, there is another problem in that the scattered light is not totally reflected by the oval curved plane. When the dimension of the light guide is increased, there is also a problem that it takes time to mold and xe2x80x9cmolding sinkxe2x80x9d is easily produced.
The primary object of the present invention is to overcome the above-mentioned problems and to provide an improved light guide in which a reading image deteriorates less even when a document surface is elevated, and an improved line-illuminating device incorporated with such a light guide.
A second object of the present invention is to provide a line-illuminating device which can generate strong scattered light even in narrow light scattering patterns by increasing density of light which is propagated through the light guide and which can allow the scattered light to be efficiently condensed on a document reading surface.
To attain the first object, according to the invention which belongs to a first group, a bar-shaped light guide is provided in which incident illumination from end surfaces is reflected by an inner surface of the light guide and emitted from an emission plane provided along the longitudinal direction, characterized in that a cross-sectional shape of the bar-shaped light guide in a direction perpendicular to the longitudinal direction is substantially xc2xc oval of which the end on a major axis side is chamfered, and a side of the bar-shaped light guide along the longitudinal direction comprises the emission plane which is parallel to a minor axis direction of the oval, a plane parallel to the major axis direction of the oval, a light scattering plane provided with light scattering patterns on the plane which is formed by chamfering the end of the major axis side of the xc2xc oval, and a reflecting curved plane for reflecting the scattered light from the light scattering patterns toward the emission plane.
The end on the major axis side of the xc2xc oval is cut (chamfered) including a focal point of the oval and a plane formed by this cutting is provided with light scattering patterns. Thus, as shown in FIG. 4, characteristics with less change in light intensity can be obtained relative to elevation of a document in a range where displacement in a sub-scanning direction is 1-2 mm. Accordingly, if an optical axis of a rod lens and a light-receiving surface of a line image sensor are arranged in an area in which the light intensity change is less relative to the elevation of the document, it is possible to reduce deterioration of a reading image even when the document surface has been elevated.
A line-illuminating device incorporated with the bar-shaped light guide is provided, in which the bar-shaped light guide is housed in a casing to allow the emission plane to be exposed, and a light-emitting means is provided on at least one end of the bar-shaped light guide. The light scattering patterns are formed in such a manner that a forming area of the light scattering patterns is larger from one end on which the light-emitting means is provided, toward the other end.
A contact-type image sensor, including the line-illuminating device incorporated with the bar-shaped light guide and a lens array for allowing reflected light from the document contained in the illumination from this line-illuminating device to be condensed toward a line image sensor consisting of a photoelectric conversion element, is provided, in which, when the lens array is formed by a plurality of rod lenses, the optical axis of the lens is arranged in an area where the change of light intensity is less relative to the elevation of the document.
With this construction, it is possible to improve decrease of light quantity resulting from the elevation of the document.
According to the invention which belongs to a second group, the line-illuminating device is provided with a light guide section for guiding light from a light source incident from end surfaces in the longitudinal direction, allowing the light to be scattered by light scattering patterns formed along the longitudinal direction, and allowing the light to be emitted from an emission plane formed along the longitudinal direction, and with a light condensing section for allowing the light emitted from the emission plane of the light guide section to be condensed on the reading surface of the document. The light guide section is arranged in contact with the light condensing section or the two are closely arranged.
The line-illuminating device according to the invention which belongs to the second group is constructed to divide the conventional light guide into the light guide section and the light condensing section. Accordingly, it is possible to narrow the cross-sectional area of the light guide section to improve density of light which is propagated through the light guide section, thereby improving intensity of the scattered light. In this case, the light condensing section is only required to have a function for allowing the scattered light emitted from the light guide section to be condensed on the document reading surface. Accordingly, the size and shape of the light condensing section can be designed taking only condensing efficiency into consideration. As a result, it is possible to improve the condensing efficiency.
It is desirable that the light condensing section be provided with a reflecting curved plane (i.e. an oval plane) which is caused to reflect light emitted from the emission plane of the light guide section and allows the reflected light to be condensed on the reading surface of the document. With provision of the reflecting curved plane (i.e. the oval plane), it is possible to allow the scattered light emitted from the light guide section to be efficiently condensed on the document reading surface.
Further, the light guide section and the light condensing section can be covered by the light guide casing except for the emission plane for the document illuminating light. By covering the light guide section and the light condensing section with the light guide casing, it is possible to allow the light emitted outside to be reflected by the light guide casing and to return the reflected light to the light guide section and the light condensing section. With this construction, it is possible to reduce loss of scattered light and improve the intensity of the document illuminating light.
Still further, the light source can be provided on only one side of the light guide section. In this case, the reflecting means can be provided on the other side. If the reflecting means is not provided, the density of the light scattering patterns which are formed along the longitudinal direction of the light guide section can be gradually increased toward the other end of the light guide section.